Typically, tape drive systems provide tape guides for controlling the lateral movement of the tape as the tape is moved along a tape path in a longitudinal direction across a tape head. The tape may have a plurality of data tracks extending in the longitudinal direction, and the tape drive system may provide a track following servo system for moving the tape head in a lateral direction for following lateral movement of the longitudinal tracks as the tape is moved in the longitudinal direction. The track following servo system may employ servo tracks on the tape which are parallel to the data tracks, and employ servo read heads to read the servo tracks to detect position error and thereby position the tape head at the data tracks and follow the data tracks. This allows the data tracks to be placed closely together and increase the number of data tracks.
The tape is typically contained in a cartridge of one or two reels, and the tape is moved between a supply reel and a take up reel. The reels typically have runout causing the tape to move laterally as the tape is moved longitudinally. Tape guides can to an extent limit at least the amplitude of the lateral movement of the tape with the goal of limiting the lateral movement so that it does not exceed the lateral movement capability of the track following servo system.
In functions other than tape guiding, such as a tension roller (U.S. Pat. No. 4,310,863), an inertia roller (U.S. Pat. No. 4,633,347), or a tape timer roller (U.S. Pat. No. 3,037,290), where only longitudinal motion of the tape is concerned, high friction rollers that are in the tape path and displaced a considerable distance from the tape head, are intended to prevent or reduce tape slippage longitudinally with respect to the roller.
Typical tape guides may comprise stationary buttons or edges, or flanges at the side of tape guide rollers, positioned against the edges of the tape to control the amplitude of the lateral movement of the tape. In order to increase the total capacity of a tape, the tape is increasingly made thinner to allow more wraps of tape to fit on a given tape reel. As a result, the tape can be relatively weak in the lateral direction, and can, in some instances, be relatively easily damaged at the edge from the tape guide. Thus, the tape guides are typically positioned at a bearing where the tape assumes a cylindrical shape, thus increasing the ability of the tape edge to support a load. The tape roller bearing is generally rotatable about a central axis parallel to the cylindrical peripheral surface, allowing the tape freedom of movement in the longitudinal direction.
The bearing is also typically designed to have low friction. This arrangement can minimize the potential to distort the edge of the tape as the guides push against the edges of the tape to move the tape to the center of the bearing to reduce the amplitude of lateral displacement of the tape. One example is illustrated in U.S. Pat. No. 5,447,279, which employs an air bearing to reduce the friction of the bearing for stationary tape guides. One type of bearing in which the tape engagement surface remains stationary may also be referred to as a fixed pin or post. Other bearings such as roller bearings may have rotating tape engagement surfaces which reduce the longitudinal friction of the bearing while the flanges of the roller bearings push against the edges of the tape. One example of a roller bearing or fixed pin with flanges arranged to have low friction is U.S. Pat. No. 4,427,166. Fixed surfaces may also be arranged to have low friction. One example is described in U.S. Pat. No. 4,466,582, where a synthetic resin or metal coated tape guide bearing has a reduced contact area for the tape to lower the friction between the guide surface and the running tape and allow the flanges to stabilize the tape.
However, when wound on a reel, tape is typically subjected to stack shifts or stagger wraps, in which one wrap of the tape is substantially offset with respect to an adjacent wrap. Thus, as the tape is unwound from the reel, there can be a rapid lateral transient shift of the tape. Other common sources of rapid lateral transient shifts include 1) a buckled tape edge in which the tape crawls against a tape guide flange and suddenly shifts laterally back down onto the bearing, 2) a damaged edge of the tape which causes the tape to jump laterally when contacting a tape guide, and 3) when the take up reel or supply reel runout is so significant that the reel flange hits the edge of the tape.
Because of the low friction of the bearing and the low mass of the tape, rapid lateral transient shift of the tape at any point of the tape path may not be slowed by the typical tape guide and thus may be transmitted along the tape path to the tape head.
A tape head track following servo system may comprise a single actuator, or a compound, multiple element actuator. The transient response of the tape head track following servo system typically comprises a high bandwidth for a very limited lateral movement, called “fine” track following, to permit the tape head to follow small, relatively rapid displacements of the tape. Larger movement of the tape head is typically conducted as “coarse” track following, which is also employed to shift the tape head from one set of tracks to another set, and is typically conducted at a slow rate. The occurrence of a lateral transient shift, however, can be so rapid that neither the fine track follower nor the coarse track follower is able to respond, with the result that the tracking error becomes so large that writing may be stopped to prevent overwriting an adjacent track and to insure that the tracking error on read back is not so large as to cause a readback error.
One approach has been to make the tape guide edges or flanges closer together to maintain a pressure on both edges of the tape. However, this tends to stress and damage the edges of the tape, reducing its durability. An attempt at reducing the stress comprises spring loaded tape guides, such as the above-mentioned '279 patent. However, although the amplitude of the tape shift may be reduced somewhat by this approach, the speed of the shift is typically not reduced, and a track following servo error may still occur, reducing the performance of the tape drive.
U.S. Pat. No. 6,754,033 describes a tape roller bearing having a cylindrical peripheral surface comprising a grooved frictional surface for contacting and engaging the surface of the tape, allowing the tape to move freely with the tape roller bearing cylindrical peripheral surface in a direction perpendicular to the central axis, and constraining movement of the tape in the lateral direction. The frictional surface limits slip in the lateral direction, thereby reducing the rate of the lateral transient movement of the tape to allow the track following servo system to follow the reduced rate lateral transient movement of the longitudinal tracks.
Thus, the tape is contacted and engaged at its surface rather than at an edge, constraining the tape in the lateral direction, providing substantial lateral drag to the tape, such that the tape is able to move laterally at a slower rate as the tape roller bearing rotates, which can substantially reduce the rate of the lateral transient movement. The grooved tape engagement surface substantially quenches any potential air bearing that could form between the surface of the tape and the surface of the roller bearing, e.g., due to the air drawn along by the tape as it is moved rapidly. As a result, an air bearing beginning to form is generally collapsed to ensure that the roller bearing frictionally contacts and engages the surface of the tape. A flat cylindrical surface may also be provided at the edges of the tape to fully support the tape edges.
Another approach has been to provide rollers having a crowned tape engagement surface which exerts a lateral force on the tape which tends to restore the tape to a central position. However, the effectiveness of this approach can be limited due to various factors such as the Young's Modulus exhibited by the tape and the degree of strain permitted to be exerted on the tape.
Yet another approach utilizes a post having a concave tape engagement surface rather than a crowned tape engagement surface. Here too, the curvature can provide some restoring force to center the tape. However, like the crowned tape engagement surface, the concave curvature is limited by the allowable tension gradient in the tape. Typically, the tension gradient is maximum when the tape is at nominal tension and the edges are “baggy” or at zero tension.
It has also been proposed to use sensors to detect the lateral position of the tape edge as it passes the bearing and to tilt the bearing in an active closed control loop to control the lateral position of the tape. It is recognized that tilting the bearing can introduce a gradient of tension between the top and bottom edges of tape which can be used to actively steer the tape riding on an air bearing formed between the tape and the physical bearing surface. However, the air bearing may be inadvertently quenched such as when the tape stops or momentary stiction occurs between the tape and the physical bearing surface. As a consequence, a momentary loss of control of the tape may be produced which may have severe consequences causing damage to the tape.